Most stock kernels are optimized by handset manufacturers for a balance between performance, stability, and battery, and they can’t be customized. Custom kernels, on the other hand, are optimized differently. They can be configured manually, and installing one can give your phone a battery boost.

Once you’ve got your custom kernel up and running, you can configure it using one of several apps from the Play Store, including the free Kernel Adiutor, or the paid EX Kernel Manager.

You may notice an immediate difference in speed, smoothness, and battery life on your phone using the custom kernel’s default settings, and it’s worth sticking with those for a while. There are options to tweak the settings further, ranging from the simple to the very technical — and mostly way beyond the scope of this article.

One easy area you can experiment with are the CPU Governors. These dictate how quickly the processor ramps up to maximum speed, and how slowly it drops back down again. Every kernel has several pre-configured governors (albeit tuned differently from one kernel to the next). These include OnDemand and Interactive, which both increase the clock speed quickly and drop down slowly (Interactive is the default on Android these days); Performance, which locks the CPU at maximum speed to complete tasks quicker; and Powersave, which locks the CPU at the minimum speed.

Experimenting with CPU Governors has an immediate and noticeable effect on performance. As with all battery life tweaks, the key is to find a balance that works for you.

Underclocking and Undervolting Don’t Extend Battery Life

Flashing a custom kernel also enables you to underclock and undervolt your processor. Both are frequently discussed as battery saving techniques, but they’re quite contentious.

Underclocking — reducing the CPU’s maximum clock speed — is largely discredited. Any gains from having the processor use less power are offset by the fact that it takes longer to complete tasks. A faster processor may use more power, but it completes tasks and then returns to an idle state much quicker.

Undervolting involves setting the processor to use a lower voltage when running at specific frequencies. It sounds compelling, but the benefits are negligible. Undervolting only works when the processor is active, and has no effect on the display, which is the biggest battery drain by some distance.

Reduce Screen Brightness

Speaking of the display being the largest drain on the battery, a simple solution to this is to reduce your phone’s brightness levels.

There are lots of apps that do this. Lux enables you to override the auto brightness levels with your own, even to the point of using a ‘sub-zero’ setting to make the screen darker than it’s allowed by default.

This might not have the desired effect, however, as Lux and other brightness apps tend to make the display darker by placing a gray overlay on the screen. The actual backlight is still using the same amount of power.

A better option is, again, a custom kernel. Most kernels have some level of display output controls, and ElementalX is among the ones that offer a ‘backlight dimmer’ function.

Doze only kicks in when the phone is completely idle. It won’t work when your phone is in your pocket — even if you haven’t turned it on or even touched it for an hour — because the phone’s sensors will detect motion. It needs to be stationary. And even, then it will wait for 30 minutes of no use before it checks for motion.

On a rooted phone, you can speed all this up with Naptime. This free app from the Play Store gives you access to the normally hidden Doze settings.

There are 16 options in total, and the app helpfully explains what each one does. You should proceed carefully, making minor tweaks each time, and checking their effects. A good place to start includes changing Inactive timeout (the amount of idle time before the sensors are checked for a lack of motion) from the default of 1800 seconds to 600 (ten minutes), and Idle after inactive timeout (an additional wait after that) to zero. That should help Doze start much sooner.

Naptime also has an Aggressive doze mode that tries to put the phone into an idle state as soon as the screen is turned off. This can potentially interfere with some apps, though, so you’ll need to keep an eye on it if you choose to use it.

Control Background Apps With Amplify and Greenify

The combination of Amplify and Greenify is one of the most effective ways to reign in excessive use of your battery.

Greenify is available in root and non-root versions, and even has some Xposed functionality through an in-app purchase.

Tackle Bloat and Resource Hogs

And finally, bloatware. Every Android device comes with numerous additional apps pre-installed, many of which you don’t want and will never use. Yet they’re there, using resources, running on boot, and generally degrading your experience.

Or, for a more comprehensive solution, find a leaner custom ROM for your phone that has had all the additional junk removed. Most mainstream devices have them, and you can notice benefits in all areas of performance.

Other Advanced Tips

There are so many different factors that can affect battery life; it’s impossible to say which solution will work best for you. Some of the ones listed above should have some effect, and they won’t disrupt your enjoyment of your phone, either.

If you’ve got any root-based battery tips, we’d love to hear them. Have you had great success with a particular kernel, or have you found an app that works wonders? Tell us all about it in the comments below.

Yes, replace them with Facebook Lite and Messenger Lite. I have the Mess Lite running in the background all the time, and it wakes up a lot of times according to the Greenify app, but still only consumes a few percent of battery during a 24 hr period.

The Facebook Lite I simply close down after use, and it seems to not have any impact on battery after that. Brilliant!

Andy is a former print journalist and magazine editor who has been writing about technology for 15 years. In that time he has contributed to countless publications and produced copywriting work for large tech companies. He has also provided expert comment for the media and hosted panels at industry events.…